1. Field of the Invention
The present invention relates to a semiconductor device and a method for fabricating the semiconductor devices and, more particularly, to a semiconductor device having a substrate made of Y3Al5O12 and a group-III nitride semiconductor layer and a fabrication method thereof.
2. Description of the Related Art
Recent years have seen a spate of active development of technologies using phosphor materials designed to obtain light emitting modules for emitting white light, for instance, using a light emitting device such as an LED (light emitting diode). For example, it is possible to produce white light by placing a phosphor material, which can be excited by blue light to emit yellow light, on an LED that emits blue light. In this regard, Japanese Unexamined Patent Application Publication No. 2006-5367 (Reference (1) in the Related Art List below) proposes a structure having a ceramic layer disposed within the path of light emitted from a light emitting layer, for instance.
For example, Reference (1) discloses bonding as a way of attaching a phosphor material, which has been formed into a sheet-like ceramic layer or the like, to a light emitting layer. However, the bonding layer is liable to degradation from exposure to the light from the light emitting layer. Also, the bonding layer can develop voids therewithin, and the presence of voids may cause a decrease in the light extraction efficiency. And, provision of a bonding layer whose refractive index is relatively low may also cause a decrease in the light extraction efficiency. Also, since the light transmissibility of the bonding layer is lower than 100%, the light passing through the bonding layer may possibly contribute to a decline in the light extraction efficiency. Further, a step of bonding is required aside from the step of allowing crystal growth of a semiconductor layer on a growth substrate. Moreover, there is need for an expensive substrate of sapphire, SiC, or the like for crystal growth thereon in addition to the phosphor material, which has been formed into a sheet-like ceramic layer or the like.
As a measure to solve the above-mentioned drawbacks, the above-cited patent publication of Reference (1) proposes a technology in which a group-III nitride nucleation layer is deposited at low temperature directly on a ceramic layer and further a buffer layer of GaN (gallium nitride) is deposited at high temperature thereon. According to this patent publication, it is possible to eliminate harmful influences of lattice mismatch by inserting multiple low-temperature intermediate layers between YAGs (see below) and the GaN buffer layers. Also, Japanese Unexamined Patent Application Publication No. 2003-204080 (Reference (2) in the Related Art List below), for instance, proposes a nitride semiconductor device in which a buffer layer is formed on a substrate formed of Y3Al5O12 of surface orientation (111) (hereinafter referred to as YAG (Yttrium Aluminum Garnet) as appropriate) and a group-III nitride semiconductor layer is formed on the buffer layer.